You might have noticed for xrdp on Debian (but quite possibly with a lot of other Linux tools and other Linux
distributions) the user limits (described in
/etc/security/limits.conf) are not enforced.
Which meant in my case that any session open with xrdp was opened with a max number of open files (
set to 1024.
To fix this, edit the file
/etc/pam.d/common-session and add the following line:
session required pam_limits.so